1. Field of the Invention
This invention relates to a process for selectively producing a linear dimer of .alpha.-alkyl styrenes in high yield by dimerizing or codimerizing .alpha.-alkyl styrenes.
2. Description of the Prior Art
It is known that dimerization or codimerization of .alpha.-alkyl styrenes (herein dimerization and codimerization are simply referred to as dimerization, unless otherwise stated) produces one cyclic saturated dimer and two linear unsaturated dimers. For instance, dimerization of .alpha.-methyl styrene produces the cyclic dimer, 1,1,3-trimethyl-3-phenylindane, the two linear unsaturated dimers, 2,4-diphenyl-4-methyl-1-pentene and 2,4-diphenyl-4-methyl-2-pentene.
Heretofore, such linear unsaturated dimers have been produced, for example, using an aromatic or aliphatic sulfonic acid or solid acid, e.g., silica gel or silica-alumina, as a catalyst (Japanese Pat. No. 6335/66), by adding a divalent or polyvalent alcohol to a reaction system containing a solid acid or cation exchange resin (Japanese Pat. No. 32845/74), or by adding a monovalent alcohol to a reaction system containing a solid acid or cation exchange resin (Japanese patent application (OPI) No. 76051/75) (The term "OPI" as used herein refers to a "published unexamined Japanese patent application"). U.S. Pat. No. 3,925,217 to Green et al also discloses preparing unsaturated .alpha.-alkyl styrene dimers by contacting the monomer with a catalyst material, such as a mixture of phosphorous oxyhalide and a small amount of a strong mineral acid. Also, disclosed therein as catalysts are acidic clays, such as activated montmorillonite and Lewis acid catalysts. These processes, however, involve many disadvantages.
The method described in Japanese Pat. No. 6335/66 which relies on the sulfonic acid catalyst requires a special reactor that prevents acid corrosion, and, in addition, if the reaction temperature is elevated to increase the dimerization rate, selectivity for the linear unsaturated dimer sharply decreases. When the catalyst, silica gel or a silica-alumina, containing 15% alumina, the method has two major defects. One defect is that the activity of the catalyst is so low that dimerization does not proceed unless the solid acid is used in an amount as high as 50 to 200% by weight based on the weight of the .alpha.-alkyl styrenes. The other is that elevation of the reaction temperature to increase the dimerization rate results in a significant drop in the selectivity for the linear unsaturated dimer.
The method described in Japanese Pat. No. 32845/74 has eliminated these defects but is complicated by the addition of water and divalent or polyvalent alcohols. The same problem is true for the method described in Japanese patent application (OPI) No. 76051/75 that adds a monovalent alcohol to the reaction system. This method also suffers from the defect that the reaction cannot be stabilized at temperatures higher than 100.degree. C. due to the high dimerization rate, consequently, the reaction must be carried out at low temperatures that slow down the dimerization rate significantly.
U.S. Pat. No. 3,994,816 describes a method of preparing a substantially hydrogenated linear dimer from .alpha.-methyl styrene, however, in this reaction, the .alpha.-methyl styrene is charged to an autoclave and the catalyst is Raney nickel.
Japanese Pat. No. 6335/66 discloses an example wherein an unsaturated dimer is selectively synthesized in the presence of a silica-alumina catalyst. The details of the catalyst composition are not disclosed except that the alumina content of the catalyst is 15 wt%. As a result of the various studies which have been conducted in attempting to achieve the above purposes of this invention, the present inventors have found that dimerization in the presence of a solid acid catalyst containing alumina and silica in a weight ratio of alumina to silica of 15/85 and which is substantially free of alkali produces a cyclic saturated dimer without the formation of linear unsaturated dimers. The present inventors have also found that if an alumina-silica catalyst that is substantially free of alkali is used, having an alumina to silica weight ratio of from 83/17 to 96/4 provides a high conversion of .alpha.-alkyl styrenes to dimers, high selectivity for the linear unsaturated dimer, and retains such high selectivity satisfactorily at temperatures higher than 120.degree. C.
Some of the so-called "silica-alumina" catalysts contain oxides of an alkaline metal or an alkaline earth metal such as sodium oxide, calcium oxide and magnesium oxide. These oxides affect the catalytic action of the solid acid catalyst and especially affect the dimerization rate. The dimerization rate sharply decreases if a silica-alumina catalyst having a weight ratio of Al.sub.2 O.sub.3 to SiO.sub.2 from 83/17 to 96/4 inclusive contains more than 2.0.times.10.sup.-4 mol/g of such oxides. Accordingly, the amount of these alkaline and alkaline earth metal oxides in the silica-alumina catalyst of the present invention must be less than 2.0.times.10.sup.-4 mol/g, and is preferably less than 0.2.times.10.sup.-4 mol/g, and most preferably less than 0.05.times.10.sup.-4 mol/g.